help diagnose old circuit board, fault

hello,

i am trying to repair a fault with an user interface circuit (UIC) board and control board out of an old 1987's computerized programmable Pfaff sewing machine (made in Western Germany).

a for fun project for me , a challenge :)

programming consists of storing values in memory locations using keys ( +/- buttons) on UIC board

PROBLEM : a group of 3 (+/-) *momentary switch* buttons on UIC board used to change memory values do not make any changes when pressed.

the UIC board has a VFD output display (16 chars) and 12 input buttons and 5 indicator LEDs. All but 3 of the buttons register presses correctly. the 3 broken buttons seem to be individually ok as i get continuity changes when i test with multimeter and push buttons.

this UIC board is connected to the control board via 12 conductor ribbon ???

I am not sure what kind of things to do to isolate trouble ........

i have dome visual inspection of solder joints and components with no obvious issues.

How would the VFD display values , LED indicatiors and inputing buttons be managed through a 12 conductor ribbon ??? in 1980's style ?? how to trace ot monitor value changes.

Any troubleshooting or tracing ideas advice would br greatly appreciated.

thanks, rob

Reply to
robb
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Well without seeing the circuit I could only guess that they used RAS and CAS to address the keypad matrix. (Row Address and Column Address).

The ribbon cable is a place to look but the addressing logic could be bad.

Tom

Reply to
Tom Biasi

They're probably worn out.

Graham

Reply to
Eeyore

and

sewing

( +/-

used

hi, thanks for reply and help.

is there a good way to test this ?

I would be convinced of that if they were all highly used keys... but there are a total of 6 micro switches (for 3 buttons +/-) and they are not typically all used with same frequency. One set is used most, one maybe half that and the others very in-frequently as the nature of the values it changes are not frequently used they are a convinience.

on the same board exists other buttons (same exact switch style) used more frequently than these and they are still working ?? if that means anything

plus continuity tests just at switch connections to board shows changes as as expected. The values may be no good but i get a continuity change that matches with known working button/switches.

I am posting pics of the circuits on the binaries schematics page if that will help ?

thanks again for your help , rob

Reply to
robb

sewing

hi Tom,

thanks for the reply and ideas. i have posted some circuit pics on the

"alt.binaries.schematics.electronic"

if that would help formulate some ideas

thanks for your help ideas and time, rob

Reply to
robb

hello,

i posted some circuit board pics on the

"alt.binaries.schematics.electronic"

under topic ---- "pfaff UIC circuit board" to

if that will help formulate some ideas on how to repair.

thanks for any help, rob

Reply to
robb

All three of them at the same time?......yeah. Besides, he said he tested them already with a multimeter.

Reply to
Anthony Fremont

What else is on the display board (part numbers)? Ca. 1987, it's probably not a dedicated micro, but there's something else active there to keep the number of wires down. I'm guessing it's probably a mechanical issue on the display/key PCB (broken trace, broken wire in the flat cable). I'd pay particular attention to the flat cable wires (test them) and the traces going to the switches.

PCB technology isn't/wasn't always 100% reliable, especially back then, and it's also possible that a via has cracked and is electrically open.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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Reply to
Spehro Pefhany

and

the machine is a Pfaff 1471 (1987 is the approx imate year it was made )

in case the year makes a difference on advice

rob

Reply to
robb

The way I would attack this problem would be: Use your ohmmeter to trace the common connections from the switches to the ribbon cable, then make sure that you have continuity through the ribbon cable. From there, trace the circuit to whatever component(s) they connect to. As another poster mentioned, they might be strobed using a CAS/RAS technique. If either signal is missing, then the circuit won't respond to set of switches. It's pretty much useless to attempt to troubleshoot your problem using the pictures you posted. You really need to get into the circuit with your ohmmeter and trace the circuitry. Draw a rudimentary diagram of your tracing. See You can also power up the boards and look at the switches with a scope. You will probably see pulses on one side of the switches. Press one of them and trace the resulting signal back through the circuit and see where it goes away. You'll have to find data sheets on the ICs involved, so plan on doing some Googling. Search for the part number of the part, and datasheet. Like this:

7401 datasheet

That should get you in the ballpark.

Cheers!!!

--
Dave M
MasonDG44 at comcast dot net  (Just substitute the appropriate characters in the 
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Reply to
DaveM

Never trust a demented donkey for repair advise.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
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Reply to
Michael A. Terrell

Isn't the title of this thread a bit misleading and unfair? It's not "all" brits - just one!

Reply to
Radiosrfun

I agree, but I wasn't the one who changed the name. It was Anthony Fremont.

OTOH, anything posted by the donkey makes his country look bad. Maybe they need to send him to the glue factory?

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
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Reply to
Michael A. Terrell

I "used" to give him some credit in the past - waited to see where he was coming from - or going to...... I'm curious - does he perhaps have a twin - we know about from elsewhere? I don't think I need to mention the name to you Michael.

I had a very brief chat (on yahoo) with another Brit who shall we say -wears the same shoes - a nut case. Well, I guess they have their share, we have ours. Maybe if we find them all through chatting, we can help the Authorities round them all up.

Reply to
Radiosrfun

The only thing that will make a difference is you posting a schematic.

Reply to
Don Bowey

well if i had a schematic i would surely post it instead of circuit board pictures.

i was hoping some one might notice the chips used and there configuration and maybe have an idea about how and what to look for.

thanks for help , rob

Reply to
robb

Three switches will not all have failed. This suggests connectivity or fault on a common line being shared.

Locate the common connection for the three switches, using simple continuity. At the same time check the other switches to see if groups share the same pattern of grouped continuity.

You should be able to follow tracks with good continuity from working switches, to get an idea where the trace for the bad row should route. When these no longer correspond, you've located a potential location to search for the fault.

If all group traces reach semiconductor pins successfully, then the first semiconductor the group hits is suspect. IO devices in machines with motor controls can suffer problems over time. These may be fairly simple and common devices (for that day), interfacing to more complex or application-specific devices.

I don't see solder-side tracking for the main board in viscinity of the connector in question. Those are a small number of wires in that harness, to handle both keyboard and display, so it may not be involved at all.

RL

Reply to
legg

you could make a list of the Ics on the board and download all the datasheets for them. That should help you.

Reply to
Don Bowey

wow thanks legg,

i guess alot of detective information is allready there in the fault description that i ignored.

namely a group of buttons have all failed yet other groups of buttons with different functions has not failed on the same board should lead one to suspect a possible common shared connection between these three failed buttons.

so look to the common connection point for all three failed buttons.

how simple and forthright yet elusive to amateurs like me.

one interesting thing i noticed is one of the alphanumeric LED cells has artifacts (slight glowing) displayed in it and the artifacts shimmer and change slightly when a BAD buton is pushed could this indicate a failure of some component ?

thanks... i will look as you suggested,the Balance key, Stitch length key and Stitch width key are the only buttons that do not seem to work.

The programming keys (ie. stitch selector buttons) do work as do all other buttons including the top right key that shows optimal settings and recommended foot. The machine will stitch the correct stitch according to parameters in the display for the selected stitch (eg. stitch 00 straight stitch at 2.5 length *and* stitch 10 zig zag at

3.0 wide 2.0 length and so on)

*problem* being that i can not change the Balance nor Stitch length nor Stitch width for any of the stitch selections 00-99 ??? when a number appears above the keys, then some stitches have the amber light and does not chnage those as expected.

I tried erasing memory as you said (no luck). I tried removing and installing new batteries (no luck).

rob

and 5

3

when i

???

Reply to
robb

Sounds like there might be a missing ground, which would cause current to flow down paths it shouldn't. It's common for problems like this to be caused by cold solder joints or hairline fractures in circuit boards, I'd start by touching up any suspect joints.

Reply to
James Sweet

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